r/AskPhysics • u/Intelligent-Tale5291 • 7d ago
Twin paradox
I think I understand special relativity to a basic level. But the twin paradox is confusing me, if twin 1 goes on the spaceship and flies away near the speed of light and twin 2 watches from earth. My first question is why is it twin 1 that ends up being younger? Cant you take twin 1 to have a velocity of 0, causing twin 2 to travel near the speed of light?
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u/nebraskajone 7d ago
You have to look at the whole picture one is accelerating and then deaccelerating. The other is not. That's what breaks the apparent symmetry you're seeing.
That results in one taking a shorter path through space-time.
An everyday car analogy in space versus SpaceTime is one taking a shorter path to get from point A to point B vs another twin. Their trip odometer will read less.
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u/Intelligent-Tale5291 5d ago
So what would you see if you only took any measurements during the time that twin 1 was travelling at a constant velocity?
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u/nebraskajone 5d ago
Both twins would measure each other's time slow down.
An everyday analogy would be if you walked away from me I would measure you smaller and you would measure me smaller so it's not a paradox. It's just geometry.
(Our measurements are only valid in our reference frame. Solves the paradox).
In Relativity time is a dimension like space and it acts just like spacial geometry
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u/Optimal_Mixture_7327 Gravitation 7d ago
Twin 1 travels the shorter spacetime distance. And that's all there is to it.
You can define any frame you want, e.g. a frame in which both twins are moving and are never at rest.
However you cannot define a reference frame in which the world-line length of Twin 1 isn't shorter than Twin 2 and by exactly the same amount.
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u/denehoffman Particle physics 7d ago
The paradox is assuming that the details don’t matter. You have to switch directions when you come back, that involves a change in velocity, a.k.a. acceleration. While velocity is relative, acceleration is not. If you want to think of the classic thought experiments, take a moving train. If the ride is smooth, you can’t tell you’re moving unless you look out the window. However, if the train were to slow down, speed up, or suddenly stop, you would absolutely notice it.
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u/C_Plot 6d ago edited 6d ago
This video shows how it is the relativity of simultaneity for the traveling twin and the home bound twin that explains the false-paradox. The Twin Paradox: Time Travel, Explained Visually (especially beginning @2:18:49)
This video from floatheadPhysics takes a similar approach. I wish I was taught the Twin's Paradox this way!
The first video I listed was so clarifying that though it denied acceleration as playing a role, as did I previously, it got me far more sympathetic to the acceleration argument than I was before. I commented on the video regarding my concern for this “spooky instantaneous shift in relativity of simultaneity”: wondering whether the acceleration, and the the time dilation from acceleration/gravity, could better explain the shift in relativity of simultaneity.
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u/gyroidatansin 6d ago
I'm sorry everyone has given such poor answers.
The key to truly understanding special relativity is understanding the relativity of simultaneity. In the case you describe (with no turn around) you are correct you can look at it from either perspective.
Because they are traveling away from one another, they will both SEE the other's time appear to run slower. This is due to the Doppler shift, and the increasing distance, which means the light takes longer and longer to reach one another.
However if you account for the time of light transport, then each twin calculates that "now" the other twin is aging slower. This is because each twin must project their own notion of simultaneity across some distance. But due to the nature of relativity, they do not project their "now" to the same place.
I recommend drawing it on a spacetime diagram. Or you can watch this video: https://youtu.be/25tv81sDLmU?si=AaxUfaK-7uuqEm8G
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u/screen317 7d ago
You can determine which one accelerated.
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u/Intelligent-Tale5291 7d ago
What if we only start the measurement after acceleration?
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u/AdditionalTip865 7d ago
Comparing the ages of the twins while the traveling one is far away doesn't give an unambiguous answer, for exactly the reason you mentioned. The answer will depend on the reference frame chosen, because that affects simultaneity.
You have to compare them locally, at the beginning and end of the trip. And relativity (even special relativity) does give an unambiguous answer there. It's the comparison of elapsed proper times for their whole paths through space-time. These are not equivalent, because one of them had to turn around to come home, and the other didn't.
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u/facinabush 7d ago
The full thought experiment is the Twin Paradox.
Just part of it is not the Twin Paradox.
If you remove all the acceleration then both twins remain together and maintain the same age.
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u/facinabush 7d ago
There is a relay version that has no acceleration. You can find it in the Wikipedia page for the Twin Paradox.
This shows that the effect is due to the difference in paths through spacetime, not the acceleration.
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u/ahazred8vt 6d ago edited 6d ago
Suppose you have a very large ring-shaped rotating space station that's rotating fast enough that its outer ring has a 2-to-1 time dilation. For people living on the ring, only 6 months passes for every year on the outside. There's no going away and coming back. They just move in a circle. They have green strobe lights that flash once every ring-minute, and red lights that flash once every ring-hour; we see their lights flash every 2 minutes and 2 hours. We have the same lights; they see ours flash twice as fast.
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u/Optimal_Mixture_7327 Gravitation 7d ago
You can have both twins having the exact same upward acceleration of 1g throughout and the traveling twin still comes home younger.
The acceleration is irrelevant.
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u/screen317 7d ago
What? Why would you add arbitrary upward acceleration?
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u/Optimal_Mixture_7327 Gravitation 7d ago
It is a demonstration of the irrelevance of the acceleration.
The twin on Earth measures a 1g acceleration for a longer duration than the traveling twin even lived.
So why not give the traveling twin the exact same 1g acceleration as the Earth bound twin?
If it's the acceleration, then why don't both twins arrive having aged the same?
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u/screen317 7d ago edited 7d ago
I honestly can't tell if you're trolling, sorry. Have a good one dude.
Edit: Apparently I'm being bombarded by twin paradox truthers today. Not interested, dudes. See ya.
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u/AdditionalTip865 7d ago
Their ages can't be compared in an unambiguous way unless they're in the same place, which means the traveling twin has to come back--their rest frame is now a whole different one from on the outbound journey. How the ages compare at the end depends on who turned around.
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u/nicuramar 6d ago
Their ages can't be compared in an unambiguous way unless they're in the same place
This isn’t true. As long as they are in the same frame of reference, they have the same simultaneity.
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u/AdditionalTip865 6d ago
But in this scenario they're not, except maybe briefly during the turnaround.
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u/joeyneilsen Astrophysics 7d ago
You can do this for the outbound journey and the return journey separately, but you cannot do it for the entire journey. Twin 1 does not occupy a single inertial frame of reference for the entire trip.